114 ISABELLA M. PT^UMMOND. 



Corresponding to this extension of the pericardium is a great 

 development of the mantle cavity to the morphological right 

 of the original right horn or kidney dnct. This is shown in 

 section with the pericardium lying above it in fig. 14, but is 

 best seen in a view of the whole animal from the left, fig. E^ ; 

 here the left horn is seen ventrally at /. m. c, the former 

 roof of the mantle cavity is almost vertical and forms the 

 posterior boundary, while the kidney duct [k. d.) is dorsal. 

 Dorsal and anterior to this is now the chief extension of the 

 mantle cavity, which is already visible in a dorsal view of the 

 whole embryo (fig. Eg), and reaching behind to the mid-dorsal 

 line, while it narrows slightly in front. Von Erlanger's 

 description of an embryo of this stage differs considerably 

 in respect of the mantle cavity from the above, but I am 

 unable to reconcile his fig. 7 on pi. xxi with my own 

 observations, for he both figures and describes (p. 358) the 

 kidney duct as arising from the dorsal extremity which is 

 now advancing over the mid-dorsal line, while a comparison 

 of the series (of which two sections are shown in figs. 13 and 

 14) with the living animal (from which the outline of the 

 mantle cavity in fig. E^ was drawn) seems to me to show quite 

 conclusively that the kidney duct arises in the position where 

 I have marked it, and that the dorsal extension of the mantle 

 cavity is a new development. If this be so, very rapid growth 

 must have taken place in the region in front of a vertical 

 lino passing through the opening of the two kidney ducts, as 

 is in fact the case, and this would lead to the increased 

 ventral flexure which attains its maximum aliout this 

 time. 



Very little remains to be said of the other organs. The 

 oesophagus is much lengthened, and has acquired a shaip 

 downward bend before entering the stomach, which slopes 

 obliquely upwards and backwards. The rectum bends sharply 

 downwards and tlien runs forward to open on the right side 

 in the inantle cavity. The right kidney has become further 

 developed and forms a simple sac, still fairly widely open to 

 the pericardium, while the left remains in much the same 



